Problem: William starts counting at 39, and he counts by fives. If 39 is the 1st number that William counts. what is the 10th number that he counts?
Answer: What is the first number that he counts? $39$ What is the second number that he counts? $\begin{align*}&39 + 5 \\ &= 44\end{align*}$ What is the third number that he counts? $\begin{align*}&39 + 5 + 5 \\ &= 39 + (2 \times 5) \\ &= 49\end{align*}$ What is the 10th number that he counts? $\begin{align*}&39 + (9\times5) \\ &= 39 + 45 \\ &= 84\end{align*}$